Friday, May 8, 2015

Tropical Storm Ana (2015)

Storm Active: May 8-11

During the first few days of May, a stationary front stalled over the northwest Caribbean sea, extending through the Bahamas. By May 5, an upper-level trough had formed over the central Bahamas, generating a widespread area of showers and thunderstorms extending into eastern Florida. A weak surface low formed in association with the trough the next day and moved close to the Floridian coast. Due to a blocking pattern situated over the mid-Atlantic states, the low moved little over the next few days, drifting only slowly northward. Meanwhile, the low deepened over the warm Gulf stream waters southeast of the Carolinas. By May 8, the system was exhibiting gale force winds in a broad region encompassing its center as it meandered a few hundred miles off of the South Carolinian coastline. Late that night, the low acquired enough convective organization to initiate advisories, but the large radius of maximum winds led to its classification as Subtropical Storm Ana.

Despite relatively favorable sea surface temperatures, dry air impeded development of convection near the center of Ana over the next day. Through most of the day May 8, almost all thunderstorm activity was confined to the southern and eastern portions of the system's circulation and the center remained isolated. This also contributed to Ana remaining a subtropical cyclone that day as it moved little. During the night, significant cloud cover finally developed near the center and the broader structure disappeared, indicating that Ana had transitioned to a tropical storm early on May 9. By this time, it had acquired a slow north-northwestward motion towards the U.S. coastline. During the day, convective banding structure improved, but Ana simultaneously began to move over the cooler water nearest to the coastline. As a result, cloud tops warmed, though the system maintained its approximate peak intensity of 60 mph winds and a minimum pressure of about 998 mb. Meanwhile, bands of rain began to move into parts of the Carolinas. Overnight, Ana weakened as it interacted with the aforementioned cooler water and land, but made landfall just south of the border between the Carolinas at 6:00 am EDT, May 10, still as a tropical storm.

The cyclone quickly weakened to a tropical depression over land and began to move toward the north and north-northeast as the ridge over the mid-Atlantic shifted eastward and diminished. By May 11, Ana was well-inland over North Carolina but still maintained its identity as a weak tropical depression. Late that night, Ana degenerated into a remnant low over eastern Virginia. The low was absorbed soon after. Ana became the second-earliest cyclone to make a landfall in the United States when it did so on May 10, behind only a cyclone of February 1952.

The above image shows Ana approaching the United States coastline on May 9, just after transitioning to a tropical storm.

Ana stalled southeast of the Carolinas due to a blocking ridge situated to its north.

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